Anharmonic Lattice Dynamics in Germanium Measured with Ultrafast X-Ray Diffraction

Damping of impulsively generated coherent acoustic oscillations in a femtosecond laser-heated thin germanium film is measured as a function of fluence by means of ultrafast x-ray diffraction. By simultaneously measuring picosecond strain dynamics in the film and in the unexcited silicon substrate, we separate anharmonic damping from acoustic transmission through the buried interface. The measured damping rate and its dependence on the calculated temperature of the thermal bath is consistent with estimated four-body, elastic dephasing times $\left(T_2\right)$ for 7-GHz longitudinal acoustic phonons in germanium.